Just some thoughts

Hi jcubie and all,

You are right coal is pretty abrasive stuff and paint wouldn't last long where its sliding past. It is also highly likely that you would have dust buildup on any flat surface and even in the nooks and crannies on surfaces that are being raised and lowered on the chute. This buildup would most likely occur from when you have dry dusty coal being sent down the chute in wet weather.

Humbrol paints do polished steel and/or gunmetal metallic finish paint. Paint either of these where the coal is moving past in the chutes and give them a buff after drying with a soft rag, polishing wheel in a Dremel or similar to bring up the abraded steel sheen inside the chute.  

Another factor is how often the chutes are in use. If regularly (< daily) then its likely that the coal/cinders would abrade the insides of the chutes to a "polished" finish. If less than daily, then some light rust would probably be likely inside the chutes. This would probably last until the next load of stuff went down the chute. If the chute was being used at less than weekly frequency then perhaps sufficient rust could build inside the chute up that might survive the next load of stuff sliding over it. 

Just some thoughts about stuff that I've seen in heavy industry,